The present invention relates to waveguide couplers and more particularly, to waveguide couplers that are efficient for coupling of light between waveguides having different shapes or dimensions. The invention is particularly useful in communications systems and in free-space optical detectors.
Optical communication systems can transmit optical signals over long distances at high speeds. An optical signal is transmitted from a light source to a waveguide and ultimately to a detector. Optical waveguide structures such as optical fibers transmit the light signals. Basically, a waveguide structure comprises an inner region (the core) fabricated from a material having a certain index of refraction, and an outer region (the cladding) contiguous to the core comprised of a material having a lower index of refraction. A light beam propagated along the core will be guided along the length of the waveguide by total internal reflection.
Planar waveguides are flat structures that guide light in the same way as optical fibers. Essentially, a planar waveguide comprises a core waveguide strip of material on a substrate where the waveguide strip has a relatively high index of refraction relative to the substrate. Thus, light is guided along the high index strip.
Optical communications systems typically include a variety of devices (e.g., light sources, photodetectors, switches, optical fibers, amplifiers, and filters). Amplifiers and filters may be used to facilitate the propagation of light pulses along the length of the waveguide.
The connections between the various system components inherently produce loss in optical communication systems. Typically, optical fibers and devices are coupled together by splicing and bonding fiber ends or by using connectors. There is a loss in transmitted light that travels from one device to the other. With the increasing demand for efficient, large-scale manufacturing of hybrid integrated opto-electronic devices, there is a need to couple many waveguide devices together while minimizing losses. For example, planar waveguide amplifiers are desirable front ends for certain detectors, and for such applications it would be desirable to couple planar waveguides with a multimode signal collection fiber.
Many other factors contribute to losses in making waveguide connections. Such factors include overlap of fiber cores, misalignment of the fiber axes, fiber spacing, reflection at fiber ends, and numerical aperture (xe2x80x9cNAxe2x80x9d) mismatch. If a fiber receiving light has a smaller NA than a fiber delivering the light, some light will enter the receiving fiber in modes that are not confined to the core and will leak out of the receiving fiber. The loss can be quantified by the formula: Loss (dB)=10 log10 (NA2/NA1)2.
Significant losses can occur if the signals are traveling from a large core into a smaller core. For example, loss of 1.9 dB can occur when light travels from a 62.5 xcexcm core fiber to a 50 xcexcm core mulitmode fiber. In some applications it would be desirable to couple light from a fiber core (having a typical dimension of 50 xcexcm to 62.5 xcexcm) to a planar waveguide or a sputtered film. However, planar waveguide amplifiers typically cannot be made with cores that are more than about 5 xcexcm thick, and conventional sputtered films have a thickness of about 2-3 xcexcm. On the other hand, a multimode signal collection fiber used in free-space optical reception systems has a core that is typically more than 50 xcexcm in diameter. This mismatch in vertical dimension makes it very difficult to efficiently couple light from a multlimode signal collection fiber to a planar waveguide. Losses can amount to up to 17 dB or in some cases up to 97 or 98 percent of the light.
As may be appreciated, those involved the development of communication systems and electro-optical devices continue to seek new designs to improve efficiency and performance. In particular, it would be advantageous to have a mechanism for addressing the vertical dimension mismatch in the cores of the components being coupled, particularly as between an optical fiber and planar waveguide.
The present invention is an optical coupler comprising a plurality of optical fibers that have unclad (core-exposed) ends and tapered cladding regions extending to cladded ends. The core-exposed ends are arranged in a bundle, and the cladded ends can be arranged as needed. The optical coupler can efficiently couple between waveguides of different core areas and shapes. For example, it may be used to connect a multimode collection fiber having a core area of greater than 50 xcexcm to a planar waveguide amplifier having waveguide strips with heights of 5 xcexcm or less.